DC-to-DC power regulator having non-linear load line and automatic leakage current offset adjustment

ABSTRACT

A voltage regulator exhibits a load line that is piecewise linear. This piecewise linearity variation has a first constant voltage segment V LEAK  at which output voltage is regulated for output currents less than or equal to the leakage current I L . The output voltage V LEAK  corresponds to the maximum output voltage allowable at the leakage current for a given operational range specification. The piecewise linearity variation further includes a second, linearly decreasing segment that varies from the maximum allowable output voltage V LEAK  at the leakage current to a full load voltage V DROOP  at full load current I FL . This serves to effectively maximize the available output voltage swing in the presence of a leakage current offset. The piecewise linear load line is adjustable to accommodate changes in leakage current.

FIELD OF THE INVENTION

The present invention relates to DC power supply systems and subsystemsthereof, and is particularly directed to a new and improved DC-to-DCconverter architecture having a piecewise linear load line thatmaximizes the available output voltage swing (V_(DROOP)) in the presenceof a leakage current offset, and which automatically accommodatesvariations in the value of the leakage current offset.

BACKGROUND OF THE INVENTION

The supply current drawn by a microprocessor, such as may be used innotebook, desktop, and system server. applications, typically variesover a relatively wide range and depends upon its activity level. Forexample, when the clock to the microprocessor is turned on, asubstantial capacitive charging and discharging current is drawn; on theother hand, when the clock is turned off, only a leakage is drawn. Noweven though there is in fact a leakage current, the power supplyspecifications of manufacturers of microprocessors often assume zeroleakage current, and a prescribed maximum current (e.g., on the order of100 amps). Moreover, microprocessor manufacturers recognize that as thecurrent switches between relatively low and relatively high currentvalues, the voltage regulator produces a transient on its outputvoltage.

This transient effect is shown in the timing diagrams of FIGS. 1 and 2.In particular, FIG. 1 shows a relatively large increase (large or fullload demand) in current I at a time t0 and a relatively large currentdecrease (negligible or no load demand) at time t1. FIG. 2 shows a‘droop’ 21 in the output voltage of the regulator that is associatedwith (and having an undershoot that slightly lags) the increase incurrent at time t0, and an overshoot in the recovery of the outputvoltage to its no load condition 22 that is associated with (andslightly lags) the decrease and return to a no load condition in currentat time t1. The droop differential must fall within the safe operatingrange of the microprocessor. The droop level 21 of the output voltage isa data integrity limit below which data can expected to be lost, whilethe upper level 22 of the output voltage is a reliability limit thatserves to avoid stressing the gates of the microprocessor.

In an effort to deal with these limits, rather than specifying that thevoltage regulator must deliver a constant output voltage, microprocessormanufacturers specify a load line wherein the regulated voltagedecreases linearly with increase in current, which allows the regulatedvoltage to vary during normal operation. This is graphically shown inFIG. 3, which depicts a nominal load line 31 midway between an upperload line specification limit 31U and a lower load line specificationlimit 31L.

Now although a load line specification is acceptable where the leakagecurrent is in the vicinity of zero amps, it becomes problematic as theleakage current increases, as it effectively ‘squeezes’ the outputvoltage range over which the converter may regulate. This leakagecurrent issue becomes especially non-trivial as semiconductormanufacturers continue to reduce line widths and thickness of gateoxides of the integrated circuits they produce, which have lead to bodyleakage and tunneling across the gate oxide.

Indeed, as graphically shown at 33 in FIG. 3, the leakage component ofthe output current can be on the order of 30%–40%, or greater, of thetotal current. This effectively means that when the current transitionsfrom a no load condition to a full load condition, as described abovewith reference to FIG. 1, the current actually transitions from asubstantial leakage current value to a full load current value, as shownat 34 in FIG. 3. The associated voltage transient response resultingfrom the load line is shown at 35 as transitioning between a leakagevoltage value V_(LEAK), which is less that the value it would have atzero leakage current, and a full load voltage value V_(FULL LOAD), thelatter being less than the leakage voltage value by the differentialvoltage V_(DROOP), as shown.

In addition to being a non-negligible parameter, the leakage current ishighly variable among manufactured part populations and may have a rangeof variation as large as three or four to one.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a voltageregulator which exhibits a load line that is piecewise linear. Thispiecewise linearity variation has a first constant voltage segmentV_(LEAK) at which output voltage is regulated for output currents lessthan or equal to the leakage current I_(L) The output voltage V_(LEAK)corresponds to the maximum output voltage allowable at the leakagecurrent for a given operational range specification. The piecewiselinearity variation further includes a second, linearly decreasingsegment that varies from the maximum allowable output voltage V_(LEAK)at the leakage current to a full load voltage V_(FULL LOAD) or V_(DROOP)at full load current I_(FL), so as to effectively maximize the availableoutput voltage swing in the presence of a leakage current offset. Inaddition, the piecewise linear load line is automatically adjusted asnecessary to accommodate changes in leakage current.

To this end, the DC converter architecture of the invention comprises anerror amplifier having a first input coupled to a reference inputvoltage V_(REF)=V_(NO LOAD). A second input of the error amplifier iscoupled to the output of a voltage summing unit. The voltage summingunit combines the DC converter's output voltage with a voltage that is afunction of the output current defined by the piecewise linear load linedescribed above. The output of the error amplifier is coupled to a powerconverter, the output of which drives a load, such as a microprocessor,whose operation is controlled by a clock signal. A stop clock line thatis common on computing platforms effectively reduces the microprocessorload to the leakage current, and the stop clock signal is used to sampleleakage current.

A current measurement probe is coupled to the output of the powerconverter and provides an output representative of the output currentbeing supplied to the load. The output current measured by probe iscoupled to a piecewise linear transform unit and to a sample and holdunit. The piecewise linear transform unit produces a piecewise linearoutput voltage that is a function of the output current, based upon thepiecewise load line characteristic and in accordance with the value ofleakage current drawn by the load. The transform unit produces a zerooutput voltage for output current values less than or equal to theleakage current, and then a linearly increasing output with increase inoutput current. The piecewise linear voltage produced by the transformunit is coupled to the summation unit.

The sample and hold unit samples the output current supplied to theload, when clocked by a stop clock signal. Because the stop clock signaleffectively disables the load (microprocessor), the value of the currentprobe's output, as sampled by the sample and hold unit, is effectivelyrepresentative of the leakage current and serves as a leakage currentoffset value that is coupled to the transform unit. The sampled leakagecurrent is held by the sample and hold unit during the interval that thestop clock is unasserted.

In operation, for a no load condition the output current as measured bycurrent probe is only the leakage current, which is controllably issampled by sample and hold unit in accordance with the stop clock inputand supplied as an offset input to the transform unit. For currentvalues equal to or less than the leakage current, the output of thetransform unit is zero. This means that summation unit will sum theoutput voltage with the zero output of transform unit, so that theoutput of the summing unit is the output voltage. With the output of thesumming unit coupled to the error amplifier, the error amplifier willdrive the power converter so as to make the regulated output voltageequal to the voltage reference voltage (V_(REF)=V_(NL)=V_(LEAK))supplied to the non-inverting input of the error amplifier. Thiscorresponds to the uppermost voltage level V_(LEAK) of the piecewiselinear load line.

For a full load condition, the output current as measured by the currentprobe is the full load current. Since this current is substantiallylarger than the leakage current, the output of the transform unit willbe a corresponding large value of voltage. This means that the summationunit will sum the output voltage with the full load voltage V_(DROOP),so that the output of the summing unit 60 will be the sum of the outputvoltage Vo (=V_(REF)=V_(LEAK)) and V_(DROOP). This will cause the erroramplifier to drive the power converter so as to make the regulatedoutput voltage equal to the voltage reference voltage(V_(REF)=V_(NL)=V_(LEAK)) minus V_(DROOP). This corresponds to thelowermost voltage level at the uppermost value of load current of thepiecewise linear characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing diagram showing a variation with time of load currentof a DC voltage regulator;

FIG. 2 is a timing diagram showing a variation of output voltage withtime in association with the load current variation shown in FIG. 1;

FIG. 3 shows a linear load line for a conventional DC voltage regulatortogether with associated output current and output voltage variations;

FIG. 4 shows a piecewise linear load line for a DC converter inaccordance with the present invention together with associated outputcurrent and output voltage variations; and

FIG. 5 diagrammatically illustrates a DC converter architecture inaccordance with the present invention for implementing the piecewiselinear load line characteristic of FIG. 4.

DETAILED DESCRIPTION

As pointed out briefly above, and as graphically shown in FIG. 4, thevoltage regulator of the present invention exhibits a load line 40 thatis piecewise linear, as shown by segments 41 and 42. In particular, theflat segment 41 corresponds to the output voltage being regulated at aconstant (no load) output voltage V_(LEAK) associated with the leakagecurrent offset value I_(L) for the upper load line characteristic 31Uspecified by the user (microprocessor manufacturer) of the DC converter.In the example of FIG. 4, the value of the leakage current I_(L) issubstantial, being on the order of one-third of the total load current.Load line segment 42 decreases linearly with increase in current, fromthe leakage current-associated voltage value V_(LEAK)=V_(NOLOAD) up to avoltage V_(FULL LOAD) at the maximum (full load) current value I_(FL).The resulting voltage transient characteristic is shown at 43.

From a comparison of FIG. 4 with FIG. 3, it will be appreciated that thepiecewise linear load line of FIG. 4 requires less output capacitance,as the upper boundary of the output voltage has been increased to theupper limit of the range of the output voltage specification, such thatthe slope (R_(LL)) of the linearly decreasing load line is steeper inFIG. 4 than for the load line in FIG. 3. Since the slope of the loadline defines the magnitude of the effective output resistance of theregulator, this means that the effective series resistance (ESR) islarger, thereby reducing the amount of output capacitance required. Forthe present example (where the leakage current is on the order ofone-third of the total current), FIG. 4 also shows in broken lines 45 asuperposition of the transient voltage response 35 of FIG. 3 and thedifference in the droop voltage V_(DROOP). Given that the outputcapacitance is limited by its effective series resistance (ESR), it canbe seen that the piecewise linear load line characteristic of FIG. 4effectively eliminates one-third of the total output capacitancerequired.

FIG. 5 diagrammatically illustrates a DC converter architecture forimplementing the piecewise linear load line characteristic of FIG. 4. Asshown therein, a reference input voltage V_(REF)=V_(NO LOAD), to whichthe regulator is referenced, is coupled to a first, non-inverting (+)input 51 of an error amplifier 50. A second, inverting (−) input 52 ofthe error amplifier is coupled to the output 63 of a voltage summingunit 60. As will be described, voltage summing unit 60 combines theoutput voltage Vo with a voltage that is a function of the outputcurrent defined by the piecewise linear load line of FIG. 4. This servesto make the output voltage follow the maximized load line characteristicof FIG. 4.

The output 53 of the error amplifier 50 is coupled to a power converter70 (which may be implemented as a pulse width modulator-based buck modeDC—DC converter) that is supplied with an input voltage V_(IN). Theoutput of the power converter 70 is coupled to an output node 73 atwhich the output voltage Vo is derived. An output capacitance Co and aload 80 are coupled between output node 73 and a reference voltage(e.g., ground). In the present example, it will be assumed that the loadis a microprocessor, the operation of which is controlled by a clocksignal therefor. Disabling or interrupting the clock serves to reducethe microprocessor load to it's leakage level. For this purpose a stopclock line 120 is employed. The stop clock signal is common on computingplatforms and serves to interrupt the sequential clocking of the centralprocessing unit. The present invention makes use of the availability ofa stop clock pin provided by the microprocessor manufacturer to measurethe leakage current, as will be described.

The output voltage Vo at the output node 73 is fed back to a first input61 of summing unit 60. (Normally, the voltage at the output node wouldbe fed back to the inverting (−) input 52 of the error amplifier 50, sothat the error amplifier would control the operation of the powerconverter in accordance with the reference voltage. In accordance withthe present invention, however, the output voltage is combined with avoltage that is a function of output current based on the piecewiselinear load line of FIG. 4, so that the error amplifier causes theoutput voltage to track that load line.

For this purpose, a current measurement probe 90 is coupled in theoutput line of the power converter 70 and provides an outputrepresentative of the output current being supplied to the load 80. Theoutput current measured by probe 90 is coupled to a first input 101 of apiecewise linear transform unit 100, and to the input of a sample andhold unit 110. Piecewise linear transform unit 100 produces an outputvoltage that is a function of the output current Io, in accordance withthe piecewise load line characteristic of FIG. 4, and in accordance withthe value of leakage current is being drawn by the load 80.

The transfer function of transform unit 100, which may be readilyimplemented using a passive diode-resistor network, is graphicallyillustrated with the block 100 as producing a fixed output voltage(e.g., zero) associated with a no load condition, for current values atinput 101 less than or equal to an offset current OFS (or I_(L))supplied to an input 102 and thereafter linearly increasing withincrease in output current. The value of the offset (OFS) at input 102of transform unit 100 is provided by sample and hold unit 110.

The piecewise linear voltage V_(PL) produced at output 103 of transformunit 100 is coupled as a second input 62 of the summation unit 60. Aspointed out above, the voltage V_(PL) is used to reduce the outputvoltage Vo as a function of load current. Sample and hold unit 110 isused to derive a sample of the output current being supplied to theload, when clocked by a stop clock signal on line 120. Because the stopclock signal effectively disables the load (microprocessor), the valueof the current probe's output, as sampled by the sample and hold unit110, is effectively representative of the leakage current and serves asa leakage current offset value (I_(OFS)=I_(L)) to input 102 of transformunit 100. Namely, the output current is sampled when the stop clock isasserted and that sampled value is held by the sample and hold unitduring the interval that the stop clock is unasserted. The operation ofthe converter of FIG. 5 may be readily understood by reference to its noload and full load conditions discussed below.

No Load Condition

For a no load condition the output current as measured by current probe90 is only the leakage current I_(L). As pointed out above, this currentis sampled by sample and hold unit 110 in accordance with the stop clockinput and supplied as an offset input to the transform unit 100. Forcurrent values equal to or less than the leakage current I_(L) theoutput of transform unit 100 is zero. This means that summation unit 60will sum the voltage Vo at output node 73 with the zero output oftransform unit 100, so that the output 63 of summing unit 60 will simplybe the output voltage Vo. With the output 63 of the summing unit 60being coupled to the inverting (−) input 52 of error amplifier 50, theoutput 53 of amplifier 50 will drive the power converter 70 so as tomake the voltage Vo at output node 73 equal to the voltage referencevoltage (V_(REF)=V_(NL)=V_(LEAK)) supplied to non-inverting (+) input 51of error amplifier 50. This corresponds to the upper voltage levelV_(LEAK) shown at 46 in FIG. 4.

Full Load Condition

For a full load condition, the output current Io as measured by probe 90is the full load current I_(FL). Since this current is substantiallylarger than the leakage current I_(L), the output 103 of transform unit100 will be a corresponding large value of voltage, here V_(DROOP) asshown at 48 in FIG. 4. (It may be noted that for a less than full loadbut greater than no load condition, the output of transform unit 100will be a voltage between V_(LEAK) and V_(DROOP).) This means thatsummation unit 60 will sum the voltage Vo at output node 73 with thefull load voltage V_(DROOP), so that the output 63 of summing unit 60will be the sum of the output voltage Vo (=V_(REF)=V_(LEAK)) andV_(DROOP). This will cause the error amplifier 50 to drive the powerconverter 70 so as to make the voltage Vo at output node 73 equal to thevoltage reference voltage (V_(REF)=V_(NL)=V_(LEAK)) minus V_(DROOP).This corresponds to the lower voltage level V_(DROOP) shown at 48 inFIG. 4.

As will be appreciated from the foregoing description, by exhibiting aload line that is piecewise linear so as to include a linearlydecreasing segment that varies from the maximum allowable output voltageV_(LEAK) at the leakage current to a full load voltage V_(DROOP) at fullload current I_(FL), the voltage regulator of the present invention isable to effectively maximize the available output voltage swing in thepresence of a leakage current offset, and thereby requires less outputcapacitance.

While I have shown and described an embodiment in accordance with thepresent invention, it is to be understood that the same is not limitedthereto but is susceptible to numerous changes and modifications asknown to a person skilled in the art, and I therefore do not wish to belimited to the details shown and described herein, but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

1. A method of providing a regulated DC voltage comprising the steps of:(a) providing a DC voltage converter, which is operative to produce aregulated output voltage; and (b) controlling the operation of said DCvoltage converter in accordance with a piecewise linear output voltagevs. output current load line characteristic, wherein said piecewiselinear output voltage vs. output current load line characteristicproduces a constant voltage for load current values less than or equalto a leakage current value, and produces a linearly decreasing voltagefrom said constant voltage to a full load voltage for load currentvalues greater than said leakage current value.
 2. A method of providinga regulated DC voltage comprising the steps of: (a) providing a DCvoltage converter, which is operative to produce a regulated outputvoltage; and (b) controlling the operation of said DC voltage converterin accordance with a piecewise linear output voltage vs. output currentload line characteristic, wherein said piecewise linear output voltagevs. output current load line characteristic produces a constant voltagefor a no load current value, and produces a linearly decreasing voltagefrom said constant voltage to a full load voltage for load currentvalues greater than said no load current value.
 3. For use with a DCconverter that is operative to supply a regulated DC output voltage to aload, and having an output voltage vs. output current load line boundaryspecification comprised of an upper load line characteristic and a lowerload line characteristic, a method of optimizing the operation of saidDC converter in the presence of a no load leakage current, comprisingthe steps of: (a) causing said DC converter to produce a constant outputvoltage, corresponding to the voltage V_(LEAK) produced by said upperload line characteristic at said no load leakage current for loadcurrent values less than or equal to said leakage current; and (b)causing said DC converter to produce a linearly decreasing voltage fromsaid constant voltage V_(LEAK) to a full load voltage V_(FULL LOAD), forload current values greater than said leakage current value.
 4. Themethod according to claim 3, wherein step (a) comprises sampling theoutput current of said DC converter during a time when no current isbeing drawn by said load, and storing a parameter representative ofleakage current in accordance with the sampled value of said outputcurrent, and generating said constant voltage V_(LEAK) representative ofsaid parameter and thereby associated with said no load leakage current,in accordance with said stored parameter.
 5. An apparatus for providinga regulated DC voltage comprising: a DC voltage converter, which isoperative to produce a regulated output voltage; and a control circuitwhich is operative to control the operation of said DC voltage converterin accordance with a piecewise linear output voltage vs. output currentload line characteristic, wherein said piecewise linear output voltagevs. output current load line characteristic employed by said controlcircuit produces a constant voltage for load current values less than orequal to a leakage current value, and produces a linearly decreasingvoltage from said constant voltage to a full load voltage for loadcurrent values greater than said leakage current value.
 6. An apparatusfor providing a regulated DC voltage comprising: a DC voltage converter,which is operative to produce a regulated output voltage; and a controlcircuit which is operative to control the operation of said DC voltageconverter in accordance with a piecewise linear output voltage vs.output current load line characteristic, wherein said piecewise linearoutput voltage vs. output current load line characteristic employed bysaid control circuit produces a constant voltage for a no load currentvalue, and produces a linearly decreasing voltage from said constantvoltage to a full load voltage for load current values greater than saidno load current value.
 7. An apparatus for providing a regulated DCvoltage comprising: a DC voltage converter, which is operative toproduce a regulated output voltage; and a control circuit which isoperative to control the operation of said DC voltage converter inaccordance with a piecewise linear output voltage vs. output currentload line characteristic, wherein said DC converter has an outputvoltage vs. output current load line boundary specification comprised ofan upper load line characteristic and a lower load line characteristic,and wherein said control circuit is operative to cause said DC converterto produce a constant output voltage, corresponding to the voltageV_(LEAK) produced by sad upper load line characteristic at said no loadleakage current for load current values less than or equal to saidleakage current, and to cause said DC converter to produce a linearlydecreasing voltage from said constant voltage V_(LEAK) to a full loadvoltage V_(FULL LOAD), for load current values greater than said leakagecurrent value.
 8. The apparatus according to claim 7, wherein saidcontrol circuit is operative to sample the output current of said DCconverter during a time when no current is being drawn by said load, andto store a parameter representative of leakage current in accordancewith the sampled value of said output current, and to cause said DCconverter to generate said constant voltage V_(LEAD) representative ofsaid parameter and thereby associated with said no load leakage current,in accordance with said stored parameter.
 9. The apparatus according toclaim 8, wherein said control circuit includes a piecewise lineartransform unit that produces an output voltage that is a function ofsaid output current in accordance with the piecewise linear transferfunction, and in accordance with said sampled and stored parameterrepresentative of leakage current.
 10. The apparatus according to claim9, wherein said DC converter includes an error amplifier having a firstinput coupled to receive a voltage corresponding to said constantvoltage and a second input coupled to receive a sum of the output ofsaid piecewise linear transform unit and said output voltage.